The present invention relates to a propeller control system, and more particularly to pitch control logic which controls a transition from forward to reverse governing.
Variable pitch propellers are employed on many differing types of aircraft having power plants ranging from piston engines to gas turbines. Conventional propeller control requires knowledge of when to deactivate the propeller speed governor and when the sense of the governor is to be inverted to provide for reverse thrust operation.
The transition from forward to reverse governing requires passage through a zone known as the bucket of the Cp curve (propeller power coefficient versus propeller blade angle for a given advance ratio) where governing is not possible. On the right side of the Cp curve (where the curve has a positive slope), the speed governor must increase propeller blade angle as input power is increased to maintain a fixed speed. On the left side of the Cp curve (where the slope is negative), the governor must do the opposite—decrease blade angle to absorb added power to maintain propeller speed.
The activation and deactivation of the governor logic along with the modification of the sense of the governor logic has been conventionally performed in an open-loop fashion by adjusting constants, timers, or rate limits to permit the passage of the propeller blade angle from the right side of the Cp curve to the left side as the propeller enters the reverse thrust range. Similar constants, timers, and/or rate limits are also required to permit the propeller to leave the reverse thrust region and pass to the right side of the Cp curve where positive thrust is again produced. This conventional open loop adjustment provides a control system which effectively transitions into and out of reverse. However, conventional systems are typically optimized for only a subset of possible aircraft operating conditions and typically possess minimal control adjustment parameters for other speeds or altitudes of operation such that the propeller system may produce unacceptable levels of propeller speed overshoot or undershoot when not operating at the optimized aircraft operating condition.
Accordingly, it is desirable to provide a propeller control system which optimizes propeller performance over the entire flight envelope.